The present invention relates to laser scanning in general and to laser scanning within inspection systems in particular.
Laser scanning involves moving a laser beam along a surface to be scanned and can be used for both writing and reading purposes. For example, laser scanning is used for writing in printing systems, where the scanned beam activates spots on a printing medium, and in cutting systems where the scanned beam cuts material. For reading, laser scanning is used in inspection systems and in copiers which use the scanned beam to illuminate consecutive spots of a surface to be viewed or a page to be copied.
FIG. 1, to which reference is now made, schematically illustrates a laser scanning system for printing and a surface 10 of a medium to be activated. The system includes a laser 12, a pre-scan optical system 16, a scan unit 14, and a post-scan optical system 20. The scan unit 14 can be an acousto-optic deflector, a polygon deflector, a hologon deflector or an oscillating mirror.
The laser 12 produces a beam 22, the pre-scan optical system 16 provides the scanned beam with the desired optical properties and the scan unit 14 deflects the beam 22 to provide the scanning motion, as indicated by arrow 24. The post-scan optical system 20 focuses the scanned beam on the medium 10, thereby to produce the printing spot, and converts the angular scan of arrow 24 to a linear scan, as indicated by arrow 26.
Due to the action of the scan unit 14, the focused beam scans a portion of the medium 10, as indicated by arrow 26, in one direction, known as the xe2x80x9cfast scan directionxe2x80x9d. The medium 10 typically is moved, as indicated by arrow 28, in a second direction, orthogonal to the fast scan direction. This is generally known as the slow scan direction. The fast and slow scan directions together provide two-dimensional scanning. Alternatively, the scan unit 14 can produce two-dimensional scanning if it includes means for deflecting the beam along a second direction.
The scanning rate (defined as pixels/sec or spots/sec) of any laser scanning system is a function of the velocity of the spot and the size of the spot, both of which are functions of the limitations of the scan unit. The scanning rate is thus limited by the fundamental parameters and quality of the scan unit. It will be appreciated that, for a given pixel or spot size, the scanning rate determines the throughput (e.g. number of pages printed or number of wafers inspected within a given period of time).
It is known to increase the throughput of a laser scanning system for printing by increasing the number of beams being scanned at one time. One such system, with 32 beams, is the ALTA-3500, commercially available from Etec Systems Inc. of California, USA.
FIG. 2, to which reference is now briefly made, schematically shows the system, but with only three beams 30. The multiple beams can be aligned along the fast scan direction, as shown, or along the slow scan direction. A beam generating unit 32, such as multiple lasers or a single laser with multiple beam splitters, produces the multiple beams 30. The multiple beams 30 pass though a system similar to that shown hereinabove for FIG. 1 but whose elements are designed for multiple beams. Thus, the scan unit and pre- and post-optical systems carry similar reference numerals as those of the scanning system of FIG. 1 but are additionally marked with an apostrophe ("").
The multiple processed beams, labeled 34, are scanned along the surface of the medium 10, thereby generating multiple parallel scan lines at one time. This typically increases the throughput of the scanning system by the number N of beams used, where an N of two to many hundreds are known.
Laser scanning systems for inspection systems utilize the scanned light for illumination of an article to be inspected by one or more detectors. Such a system is shown schematically in FIG. 3, to which reference is now made. Like the previous scanning systems, it also includes laser 12, scan unit 14, pre-scan optical system 16 and post-scan optical system 20. However, the inspection system also includes multiple light detectors 40 for detecting the shape of features on a surface 42, such as the surface of a semiconductor wafer, from different viewing perspectives. The movement of the surface 42 is indicated by arrow 44.
The scanning elements illuminate the surface 42 from above and the surface 42 scatters the light in many directions, as a function of the optical characteristics of the features thereon. The inspection system of FIG. 3 is a xe2x80x9cdark fieldxe2x80x9d inspection system since its detectors 40 collect the light scattered from the surface 42 at an oblique angle xcex2 which is outside of the convergence angle of the post-scan optical system 20.
The oblique angle xcex2 varies depending on the type of surface to be inspected and the type of features to be inspected. The light detectors 40 are typically non-imaging detectors, such as a photomultiplier tubes, which measure the changing intensity, over time, of the light impinging upon them. As is known to those skilled in the art, in order to differentiate the light from different pixels on the surface 42, the signal from the photomultiplier tube must be sampled at a rate corresponding to the spot size and to the velocity of the spot on the surface 42. This may be called xe2x80x9ctemporal resolutionxe2x80x9d.
As in the other scanning systems, the scanning rate of the inspection system of FIG. 3 is a function of the fundamental parameters and the quality of the scan unit 14. Of course, as in other scanning systems, it is desirable to increase the scanning rate of the inspection system. However, an inspection system does not easily lend itself to operating with multiple beams. One reason is that non-imaging detectors do not discern the position from which the light was scattered. Adding other beams would, therefore, cause cross-talk on the detectors caused by the signals from the other spots. Imaging detectors cannot easily be incorporated into a dark field imaging system since, due to oblique incidence angle xcex2, the collection optics cannot resolve sufficiently small pixels such as is possible with detectors placed at a non-oblique angle.
An object of the present invention is to provide multiple scanning beams within an inspection system.
There is therefore provided, in accordance with a preferred embodiment of the present invention, an inspection system using at least dark field imaging which includes a multiple beam laser scanning unit and at least one multiple beam dark field imaging unit. The laser scanning unit generates multiple beams which illuminate multiple spots on a surface to be scanned. Each imaging unit collects light from one viewing perspective and separately detects light scattered from the multiple spots.
Moreover, in accordance with a preferred embodiment of the present invention, each imaging unit includes a plurality of photodetectors, at least one per spot, spaced apart from each other and collection optics directing light scattered from each spot to an assigned one of the photodetectors.
In one embodiment, the collection optics and photodetectors are arranged according to the principles of Scheimpflug imaging. For example, the collection optics are mounted so that the longitudinal axis of its thin lens equivalent is at a first non-parallel angle to the surface, the multiple photodetectors are mounted along an image plane of the collection optics and the image plane is at a second non-parallel angle to the longitudinal axis.
Additionally, in accordance with a preferred embodiment of the present invention, the collection optics may include a spatial filter which limits the range of angles of scattered light which are received by the multiple photodetectors. The limiting unit can be an aperture stop. The collection optics can also include wavelength filters and/or polarization filters.
Further, in accordance with a preferred embodiment of the present invention, the multiple beams are separated by a separation distance which ensures that light scattered from each associated spot is received only by its associated photodetector. In one embodiment, the separation distance is generally a multiple K of the length of a scan line less an overlap amount. K can be two. The present invention incorporates other separation distances.
Finally, there is provided an inspection system using dark field imaging which includes a sensor unit which spatially separates between multiple scan lines scanned at generally the same time and which temporally separates pixels within the multiple scan lines.